1. Field of the Invention
This invention relates to a paste for filling through-holes of a printed wiring board and a printed wiring board produced by using the same. The paste of the present invention is useful in high-density printed wiring boards, especially multilayer printed wiring boards, and can be used in various printed wiring boards for information communication which are exposed to severe conditions, such as an IC package for MPU.
2. Description of the Related Art
The upper and lower wiring layers of printed wiring boards (hereinafter abbreviated as PWB) and copper-clad laminates having through-holes are electrically connected by plating the inner wall of the through-holes with metal to form a conductor layer, and the through-holes are filled with a paste. PWBs having such through-holes are disclosed, e.g., in JP-A-62-224996 (the term "JP-A" as used herein means an "Unexamined Japanese Patent Publication") and JP-A-63-137499. These wiring boards, etc. with through-holes are chiefly for general use, having found little use in IC packages for MPU that are required to be highly reliable. Ceramic wiring boards, etc. are usually used in the latter field.
In recent years, however, PWB and laminate boards having through-holes have been extending their use in IC packages for MPU in order to cope with growing demands for increasing the wiring density and the number of wiring layers to be laminated. Where a chip is mounted on an IC package for MPU with its through-holes filled with a conventional paste for general use, cracks 51 tend to develop in a built-up layer 5 on the through-hole 2 as shown in FIG. 5. Such crack development in a built-up layer is generally considered attributable to the difference in thermal expansion between a wiring board 1 and a paste 4.
The inventors have tested a number of pastes having a varied coefficient of thermal expansion but failed to inhibit crack development in a built-up layer. Thinking that the cause of cracking lies in something else, the inventors conducted extensive investigations. As a result, it has been found that the crack development is caused by shrinkage of a hardened paste in a through-hole. That is, they have found that a hardened product of a paste obtained in a filling step undergoes shrinkage on further hardening in a solder reflow step, and the shrinkage remains after cooling to generate a tensile stress in the thickness direction of the built-up layer. Hereinafter the product obtained by hardening a paste in a filling step will be referred to as a first cured resin or a first curing product, and the further hardened product resulting from heating the first cured resin in a solder reflow step will be referred to as a second cured resin or a second curing product.
In order to improve performance of electronic equipment from the aspect of PWB, studies have been given to application of a photolithographic technique for achieving high-density wiring and a build-up process for making a multilayer wiring structure. Where a conventional PWB having through-holes is used, wiring cannot be formed on the opening of through-holes, and the circuit design has restrictions such that the circuit must take a circuitous route, which has been a bar to achievement of high-density wiring and multilayer wiring structure.
To achieve high-density wiring and an increased number of built-up layers while overcoming the above problem, a method comprising filling a through-hole 2 of a PWB 1 with a resin paste 10 and forming wiring even on the through-hole and further building up an insulating layer 6 has recently been developed and is attracting attention (see FIG. 3). The resin paste used contains a filler 5, usually an inorganic filler such as silica, to suppress thermal shrinkage on curing (see JP-A-2-284951). Further, the inner wall of the through-hole 2 is plated to form a copper deposit layer 3 for electrically connecting the upper and lower wiring layers (see JP-B-5-28919). In such a PWB structure, where wiring is made by an additive process and an insulating layer is built-up thereon, a via-hole 7 cannot be formed on the through-hole 2 and should be made aside from the through-hole 2 as illustrated in FIG. 3.
It has now come to necessary to form via-holes on through-holes so as to satisfy the ever-increasing demands for high-density wiring and a multilayer structure. To realize this, copper should be deposited on through-holes, too. However, a conventional paste comprising a resin and an inorganic filler such as silica has poor adhesion to a copper deposit layer formed thereon after it is stuffed into through-holes and cured. As a result, as shown in FIG. 4, the copper deposit layer 3 tends to separate from the cured resin or undergo blistering to produce a void 32 between the copper deposit layer 3 and the cured resin. Besides, the adhesion between the resin and the inorganic filler particles is also insufficient, which tends to result in formation of air bubbles in the cured resin.
On the other hand, a conductive paste comprising a resin and a metallic filler, such as copper or silver, has been proposed (see JP-A-8-311157, JP-A-7-14427, JP-B-8-21254 (the term "JP-B" as used herein means an "Examined Japanese Patent Publication"). While the conductive paste offers improvement in adhesion between a cured resin and a copper deposit layer, it still fails to sufficiently inhibit separation and blistering of the copper deposit layer on through-holes, and the problem remains particularly for uses where high reliability is required under severe conditions. In addition, cases sometimes occurs in which the cured resin separates from the copper deposit layer formed on the inner wall of through-holes to make gaps.
Moreover, the conductive paste containing a metallic filler, while achieving improved adhesion to a copper deposit layer, has lower flowability than a resin paste containing an inorganic filler such as silica. Therefore where through-holes are filled with the conductive paste by screen printing or a like technique under the same conditions as adopted to an inorganic filler-containing paste, resin starvation may tend to occur. While not clear, the resin starvation seems attributed to greater friction among metallic filler particles than among inorganic filler particles such as silica due to some action, for example, of mechanical properties or surface energy.
Addition of a reactive or non-reactive diluent to the metallic filler-containing paste could decrease the viscosity and thereby increase the flowability. However, addition of a reactive diluent tends to result in reduction of heat resistance of the paste or crack development in a built-up layer. Where a non-reactive diluent is added, the paste tends to largely shrink on curing, which will result in separation from the copper deposit layer formed on the inner wall of through-holes to make a gap. In some cases the copper deposit layer formed on through-holes may peel or blister.
Cracking in a built-up layer as referred to above can take place irrespective of whether the paste contains a metallic filler or an inorganic filler. As previously elucidated, the cracking in a built-up layer is due to a tensile stress generated in the thickness direction of the built-up layer by shrinkage of once cured resin in a solder reflow step. Cases are also met with depending on the composition of the paste, particularly the composition of the matrix resin, in which the first cured resin becomes harder and brittle in the solder reflow step and suffers cracks. Cracks in this second cured resin can also develop in reliability tests, such as a cycling test.
The PWB having through-holes is basically produced through the following procedure.
Through-holes are pierced through an insulating substrate or a copper-clad laminate, and the entire surface of the substrate inclusive of the inner wall of the through-holes is plated electrolessly to form a copper deposit layer. A resist layer is then formed thereon, exposed to light, and developed to form a wiring pattern. The wiring area and the inner wall of the through-holes are plated with metal to a prescribed deposit thickness. The resist layer is stripped, and an unnecessary area of the metal deposit layer is removed by etching to form a circuit.
Then the through-holes are filled with an insulating paste by screen printing, etc., and the paste is heated to cure to obtain a PWB. The insulating paste frequently used comprises a heat-resistant epoxy resin and an inorganic filler so as to have a minimum difference in thermal expansion from the wiring substrate. An insulating layer called a built-up layer and a wiring layer are then successively formed thereon by a known build-up process to produce a multilayer PWB (ML-PWB).
Electronic elements such as an IC chip are mounted on the ML-PWB thus produced in a solder reflow step, in which the ML-PWB is heated at about 270.degree. C. for about 10 minutes in a solder reflow oven, followed by cooling to room temperature. In this cooling stage or subsequent reliability tests, cracks are liable to develop in the built-up layer laminated on the through-holes. Such crack development in a build-up layer is generally accepted to be attributable to the difference in thermal expansion between the wiring board and the paste.
The inventors of the present invention tested various kinds of pastes having a small difference in thermal expansion from the wiring board but failed to inhibit crack development. Further, where the curing temperature of the resin paste in a filling step is too high, there arises a problem that the cured resin itself after a solder reflow step suffers cracks through the thermal history during reliability tests, such as a cycling test, which are carried out in a simulation of the working environment of the equipment in which the PWB is incorporated.